High electrical stability and low process and refire sensitivity are critical requirements for thick film resistor compositions for microcircuit applications. In particular, it is necessary that the resistance (R) of the films be stable over a wide range of temperature conditions. Thus, TCR is a critical variable in any thick film resistor composition. Because thick film resistor compositions are comprised of a functional or conductive phase and a permanent binder phase, the properties of the conductive and binder phases and their interactions with each other and with the substrate affect both resistivity and TCR.
Since copper is an economical electrode material, there is a need for thick film resistor systems which are compatible with copper and fireable in a nonoxidizing atmosphere and which have properties comparable to air fired resistors. Among the resistance materials which have been suggested for this purpose are lanthanum hexaboride, yttrium hexaboride, rare earth hexaborides and alkaline earth hexaborides. In this regard, Baudry et al. in French Patent No. 2,397,704 have suggested resistance materials which are stable in a nonoxidizing firing atmosphere comprising an admixture of finely divided particles of a metal hexaboride and a glass frit which is an alkaline earth metal boroaluminate. In the Baudry patent, it is disclosed that the glass, which does not react with metal hexaborides, may contain no more than about 1% by volume metal oxides which are reducible by the metal hexaboride. Furthermore, in applicant's EPO Patent No. 0008437 are disclosed resistance materials which are comprised of an admixture of finely divided particles of metal hexaboride and a glass which is not reducible by the metal hexaboride. In this patent, it is disclosed that the glass may contain no more than 2 mole % of reducible metal oxides. In addition, U.S. No. 4,225,468 to Donohue is directed to similar hexaboride resistance materials comprising an admixture of finely divided particles of metal hexaboride, nonreducing glass and various TCR modifiers dispersed therein in particulate form, including particles of TiO and NbO.
Izvestia Vysshikl Uchebnykl Zavendenii, Nefti y Gaz, 16 (6), 99-102 (1973), discloses thick film resistors based on relatively coarse LaB.sub.6 and borosilicate glass. These resistors are said to be resistant to hydrogen gas; however, the films are moisture sensitive.
British Patent No. 1,282,023, published July 19, 1972, discloses electrical resistor dispersions containing rare earth or alkaline earth hexaboride conductive pigment and a glass phase dispersed in ethyl cellulose medium. The glasses used are lead borosilicates as well as lead aluminoborosilicates, the latter of which is shown to contain as little as 16 mole % of hexaboride reducible oxides of low melting metals such as Pb, Na, Co and Ni. While such metal hexaboride-based resistors have been found to be quite useful, they nevertheless have also been found to be somewhat limited in their power handling capability, especially when they are formulated to make resistance materials in the 1K-100K ohm range. More recently, Francis-Ortega in U.S. Pat. No. 4,420,338 discloses resistors of metal hexaborides containing alkaline earth silicoborate glasses modified with small amounts (less than 5 mole %) of reducible oxides of V, Nb and Ta. The purpose of the reducible oxide is purported to be to improve TCR. However, it has been found that such oxides react with the hexaborides to form either diboride particles or metals which progressively lower the resistance. This process instability is shown by excessive lowering of the resistance on refiring.
More recently in allowed copending U.S. Patent Application Ser. No. 581,601, filed Feb. 21, 1984, now U.S. Pat. No. 4,512,917 disclosed improved hexaboride resistance materials having better power handling, electrical stability, process sensitivity and refire characteristics containing metal hexaboride, and crystallizable glass having at least 5 mole % Ta.sub.2 O.sub.5 dissolved in the glass. However, in applications where these materials were used with tungsten-containing copper terminations, it has been found that the resistance tends to drift, especially at resistance levels of about 10K. For example, on aging at 150.degree. C. and/or upon being exposed to high humidity, the resistance of the prior art resistors tends to increase. Moreover the resistance tends to drop when the material is subjected to an overload voltage.